The long-range goal of this application is to understand basic photochemical and photobiological processes needed to minimize drug phototoxicity responses and to design new drugs and approaches for rational application of membrane photosensitization in medicine and biology. This application has two specific aims: (1) To relate reactive species in membrane photosensitization and their site of formation to biochemical changes produced in the cell plasma membrane and to cell functional changes. This fundamental information can be used to design dyes for photochemically initiated treatments and for explaining the effects of phototoxic drugs. Membrane photosensitization involving singlet oxygen or aryl/halide radicals produced from phototoxic drugs will be compared by measuring changes in plasma membrane potential, active transport, and membrane permeability in keratinocyte and myelomonocytic cell lines. The influence of photosensitizer location will be determined by creating the same reactive species within and outside of the membrane and by creating different reactive species at the same locations within membranes. The relationship between mechanism for membrane damage and alteration of membrane-mediated cell functions will be assessed for photosensitized inhibition of mast cell degranulation, release of arachidonic acid by keratinocytes, and inhibition of antigen presentation. (2) Determine the mechanisms by which high intensity laser radiation can influence the efficiency of membrane photosensitization. Use of high intensity radiation can alter photosensitization efficiency and introduce nonoxygen-dependent mechanisms. These effects may be useful in photodynamic therapy of hypoxic regions of tumors and other photosensitization treatments. The relationship will be established between radiation intensity and the yield of photosensitizer excited states, the subsequent absorption of a photon by the excited states, and the yields of reactive species. Photophysical studies will employ Rose Bengal and merocyanine 540 in liposomes and RBC membranes. The ability to predict the efficiency of intensity-dependent photosensitization from photophysical measurements will be tested. Photobiological studies will measure photosensitized inhibition of RBC membrane enzyme activities and cytotoxicity to a leukemia cell line using high laser radiation intensities.